Composite transmission shaft joint

ABSTRACT

A fibre reinforced composite shaft bearing a metallic flanged end coupling (F 1 ) attached to the outside diameter through a concentric cylindrical torsional joint, comprising: a)—a cylindrical end region comprising a wedge shaped inner layer of fibre (Hp) composite; b)—a layer of outer helical composite plies (He) forming the shaft and extending over the wedge shaped inner layer, wherein the layer of outer helical plies has a tapered end part overlying wedge shaped inner layer to form the cylindrical end portion, wherein all helical plies of fibre layers are exposed on an outer surface; and c)—a metallic flanged end coupling attached to the outer surface of the cylindrical end region through a primary mechanically interface (Sp) which may be splined. The joint may be strengthened by internally reinforcing the main shaft with an interference fit tubular plug (Pg) and protected from environmental degradation by inboard secondary adhesive bond (Ad).

This invention relates to a means of producing a lightweight compositetransmission shaft with metallic end fittings attached through a splinedmechanical interface in each joint end. Such shafts for use in torquecarrying, power transmission applications are highly dynamic as in motorpropshafts, marine shafts, aircraft flap shafts, helicopter driveshafts, industrial drive shafts, wind turbines and dynamometers. Theyare required to have good torsional, static and fatigue strength coupledwith a high whirling resistance. To achieve the latter, low shaftdensities, large diameters, reduced length and high longitudinal modulusare all advantageous characteristics. However, for any specific designapplication the lengths and diameters of the shafts are fixed. Amaterial combination with high specific axial modulus (high longitudinalmodulus and low density) is required to produce a shaft with highresistance to whirling. To achieve this, composite tubes reinforced withhigh modulus fibres and in particular high modulus carbon fibrereinforced plastics (CFRP) are the materials of choice. Torque istransferred through flanged end fittings attached to the shaft ends. Astructurally efficient design of this joint mechanism is the subject ofthis invention.

Fiber reinforced composite shafts exhibit advantages over metallicshafts, i.e., they are lighter in weight, more resistant to corrosion,stronger, and more inert. Fibre reinforced drive shafts comprising bothglass fibers and carbon fibers in a resinous matrix have been disclosedin U.S. Pat. No. 4,089,190, “Carbon Fiber Drive Shaft” by Worgan andReginald. Tubular fibre reinforced composites have been proposed, asdemonstrated by U.S. Pat. Nos. 2,882,072 issued to Noland on Apr. 14,1959, and 3,661,670 issued to Pierpont on May 9, 1972, and in BritishPat. No. 1,356,393 issued on Jun. 12, 1974. Vehicle drive shafts fromtubular fiber reinforced composites, as demonstrated by U.S. Pat. No.4,041,599 issued to Smith on Aug. 16, 1977, and to Rezin and Yates(Celanese Corporation) in U.S. Pat. No. 4,171,626. Here the filamentsbearing an uncured thermosetting resin are wound around a mandrel untilthe desired thickness has been established, whereupon the resinousmaterial is cured. Zones or layers are positioned circumferentiallywithin the wall of the shaft in the specific angular relationships theredisclosed. The transmission of torque into the composite shaft throughmechanical and adhesive joints is the subject of a series of furtherCelanese U.S. patents granted in 1980-1981: U.S. Pat. Nos. 4,185,472,4,187,135, 4,214,932, 4,236,386, 4,238,539, 4,238,540, 4,259,382 and4,265,951. Mechanical fixing of a tubular composite shaft through aninternally fitted tubular metallic splined interface is described inJP2001065538 by Manabu et al (Mitsubishi Motors Corp.)

Composite shafts can be manufactured in a variety of ways. Filamentwinding allows combinations of winding helix angles, ply thicknesses andfibre type to be used in optimised lay ups. The main shaft may be madefrom fibrous reinforcement in a polymeric matrix. The fibres may bebased on carbon, glass, ceramic or high stiffness polymer filaments orfrom hybrid mixes of these fibrous forms. The matrix may be based onthermosetting polymers such as epoxy or for high temperatureapplications polyimide or bismaleimides. Production methods can be basedon laying combinations of low angle helical, higher angle helical andhoop oriented layers distributed throughout the tube thickness to givecombinations of controlled wall section, torsional and longitudinalstiffness and strengths commensurate with the design requirements. Thecomposite tube properties are tailorable through control of the relativethickness of the plies and angles relative to the axis of the shaft.Fibres wound at low angles <30° impart high axial tensile properties;fibres wound at 40-50° impart high torsional properties; fibres wound at75-89° impart high hoop properties.

The present invention relates to the modification of the compositelaminate in the end regions of the main shaft over which the load is tobe transferred between the composite and the flanged end fitting suchthat the load to be evenly distributed into the torque bearing plies ofthe tube. The present invention accordingly provides methods andapparatus as defined in the appended claims

Certain embodiments of the invention will now be described, by way ofexamples only, with reference to the accompanying drawings.

FIG. 1 is a view of the as-wound end of a filament wound composite tubewhich forms the basis of the main shaft.

FIG. 2 shows the end of the composite main shaft after machining of theend region.

FIG. 3 shows the details of a hoop stiffened composite end plug which isdesigned to be push fitted into the ends of the main shaft.

FIG. 4 shows the composite inner cylinder interference fit.

FIGS. 5 a-c shows the details of an internally splined metallic endcoupling.

FIG. 6 shows a section of the main shaft with a splined end pushed onand mechanically locked in place through a high interference fit withthe serrations of the end fitting.

FIG. 7 shows an end cross-section of the assembled shaft with anadhesive bonded joint in board of the mechanical splined joint.

The present invention relates to the means of forming a lightweightstructural connection between a fibre reinforced composite shaft and ametallic end flanged coupling. The attachment is made to the outsideside diameter of the composite through a concentric cylindricaltorsional joint which consists of a primary mechanical joint which mayincorporate serrated splined internal features and is designed to beable to withstand both high torsional and axial tensile and compressiveloads and is not reliant on an adhesively bonded joint. The design alsoallows the optional use of a secondary load path accommodated throughstructural adhesive bond which also forms a protective barrier in thejointed region against the operational environment. This designphilosophy is very important in aerospace shafts where NDT methods arenot able to determine accurately the integrity of a bonded joint. Thismakes primary adhesive joints unacceptable in many aerospaceapplications.

The present invention relates to the modification of the compositelaminate in the end regions of the main shaft over which the load is tobe transferred between the composite and the flanged end fitting suchthat the load to be evenly distributed into the torque bearing plies ofthe tube. This is achieved by using a wedge shaped inner layer of highangle hoop fibre to control the local through thickness geometry of theouter helical plies such that after a simple machining operation on theouter surface of the composite tube in the end regions, the pliesthrough the tube wall thickness are projected onto the surface of thetube. The primary load transfer mechanism is then made through anexternal interference fit with the modified surface of the compositetube and a metallic fitting which may incorporate serrated splinedinternal features. This forms a mechanical interface with each helicalply layer.

An optional secondary load path can also be accommodated through anin-board adhesive bond to the outside diameter of the composite. Theends of the bonded region are designed to accommodate adhesive filletswhich reduce stress concentrations at the ends of the stressed joint.The presence of the adhesive gives both increased stability to the jointand acts as a sealant to encase all of the exposed fibres on the endface of the tube and serves to protect the exposed fibres from theenvironment.

FIG. 1 shows a composite tube illustrating impregnated fibre layerswound on a former mandrel to form a shaft according to the embodiment ofthe invention. The main shaft composite construction consisting of amultiplicity of layers of reinforcement at distinct winding angles (±a₁,±ao) to form the main shaft wall. The fibre reinforcement in each layermay be of the same type or may vary between the layers to allow a widerrange of tailorable properties of the composite tube to be achieved.FIG. 1 illustrates a two angle construction, however, any number oflayers and winding angles may be used in practice such that the requiredmechanical properties of the main shaft with inside diameter (di),outside diameter (do) and total composite wall thickness (tc) is builtup. The thin inner layer (Ho) is wound at +ao to the axis C/L. The angleao is preferably 85-89.5° so that the fibre of the layer Ho are close toa hoop orientation as are the end regions which are wound at ±ao in anumber of layers of differing lengths to give a distinct taper angle ofα with respect to the axis C/L. The outer helical layers (He) are woundover the mandrel length producing a build up of composite thickness overthe end regions.

The various fibre layers are impregnated with uncured resin and laid asshown in FIG. 1. The resin is then cured by any known method. Aftercuring the matrix of fibre layers and resin to a composite material, thetube is cut to length and the ends of the composite are machined to givea parallel end section as shown in FIG. 2. This machining operation cutsthrough and projects the edges of the helical plies running over thehoop wedge region of length Lw. The exposed edges of all of the helicallayers in the machined cylindrical hoop wedge region (Lw) produces anoptimized surface though which torsional stresses can be transferredwithout premature shearing of individual helical layers.

FIG. 3 shows the construction of composite end plugs (Pg) which areformed onto a mandrel of diameter IDp in a multiplicity of filamentwound layers of reinforcement at distinct winding angles (±a₂, ±a₃). Thefibre reinforcement in each layer may be of the same type or may varybetween the layers to allow a wider range of tailorable properties ofthe composite tube to be achieved. FIG. 3 illustrates a four-layerconstruction, however, any number of layers and winding angles may beused in practice to give the required balance of hoop to axial stiffnessand strengths. The plugs are machined to length (Lp) and on theirexternal diameter (ODp) to give a light interference fit with the innerdiameter of the main shaft. One end is lightly chamfered (Ch) at theouter diameter to enable the subsequent insertion into the end of themain shaft.

FIG. 4 shows one end of the main shaft with the plug (Pg) inserted justpast the wedge length distance (Lw). The plug increases the hoopstiffness and strength of the end regions to prevent material in themain shaft wall buckling during the subsequent interference fitting ofthe metallic ends. The interference fit of the plug makes it possible toput equal and opposite radial forces through the torque carrying elementof the tube from the plug as the assembly of a serrated fitting iscarried out.

FIG. 5 a-c shows details of the metallic end fittings. FIG. 5 a shows alongitudinal section of the splined end fitting. FIG. 5 b shows an endview of the flange and FIG. 5 c shows the details of the splined teeth.The fittings consist of an internally splined region (Sp) of length Ls,a cylindrical unsplined region (Sa) of length La, a flange region (F1)containing a number of bolt holes (Bh), distributed evenly around acommon pitch circle diameter, through which the torque can betransmitted into and out of the shaft. FIG. 5 b shows a flange with sixboltholes; however, any number of holes may be used depending on thestresses to be transmitted. The spline consists of a multiplicity ofteeth (Te) with inner diameter just less than the machined outerdiameter of the main shaft. A particular design of teeth is shown inFIG. 5 c. The teeth points at the inner diameter subtend an angle around90°; the spline shape at the outer diameter is smoother. Combinedtogether these features give low torsional stress concentrations in themetallic splined joint.

FIG. 6 shows the assembled primary mechanical joint. The spline teeth(Te) have an inner diameter just less than the machined outer diameterof the main shaft such that when the splined metal ends are axiallycompressed onto the main shaft a matching spline is cut into thecomposite surface as the heavy interference fit is formed over a lengthLs. Ls is designed to be greater than Lw such that mechanical locking toall the main shaft helical plies occurs in order to maximise thestrength of the mechanical joint. This end attachment process isinherently self-aligning giving highly concentric outer machineddiameter of the main shaft ends and the flanged end fittings. Themechanically interlocked splined length (Ls), number of teeth and theirgeometry is chosen so that the mechanical interlock is able to supportthe required ultimate compressive and tensile loads as well as beingable to transmit both design static and cyclic torque. The end region(Sa) defined by length La with a small radial clearance remains unbondedin the dry assembly process. When the splined fitting is pressed overthe outside it transmits compressive stresses into the shaft walls andthis can lead to combined compressive loads and torsional loads in theshaft during use which could reduce the torque carrying capacity of thepart. By optimising the interference fit of the plug it is possible toput equal and opposite radial forces through the torque-carrying elementof the tube from the plug as from the serrated fitting. This means thatan extremely high integrity joint can be made without introducingcombined hoop and torsional loads into the torque carrying elements ofthe tube. In order to achieve this it is desirable that the compositeplug actually preloads the end of the tube hoopwise by putting inpositive hoop stresses so that the negative hoop stresses imposed by theassembly of the end fitting onto the tube cancel these out leaving thehelical plies in their ideal state to maximise the torque carryingcapacity of the tube.

FIG. 7 shows the finished shaft which has been assembled as describedabove except that a high strength paste adhesive or sealant (Ad) isincorporated into the joint area around the serrated fitting. Theadhesive forms a structural joint over the region La between the endfitting and the composite tube through a multiplicity of composite plyinterfaces machined in the main shaft wall section (He). This increasesthe strength of the joint region and also acts to seal the mechanicalinterface from the long-term effect of the working environment. Theadhesively bonded region (La) acts as a secondary load path in thejoint. The adhesive can be applied prior to assembly or it can also beintroduced after dry assembly using a vacuum potting technique or aninjection process. The adhesive sealant also fills the gaps between thefittings and the machined surface of the composite which adds a radialconstraint to the machined surface of the composite preventing thesurface of the composite form buckling or peeling away ensuring that theload path for the torsional load is through shear of the composite incontact with the teeth of the spline. The ends of the adjacent jointregion E1 and E2 are shaped to accommodate adhesive fillets which act toreduce the stress concentrations at the ends of the adhesive joint. Therelative length ratio Ls/La can be adjusted to alter the balance betweenthe stresses carried by the mechanically fixed to adhesively bondedsections of the joint.

As first example of an embodiment of the invention the properties of acomposite transmission shaft will now be illustrated based on sizes andply orientations suitable for an aircraft wing flap lift shaftapplication. Here 30,000 cycle fatigue torques of up to ±245 Nm androtation speeds of 1300 rpm are a typical test requirement. Thedimensions of the composite shaft would have an outside diameter of 33mm with an internal diameter of 27.7 mm and length of 1.75 m. Axialcompressive loads in excess of 7 kN can be supported with less than 5 mmtransverse deflection. To achieve this, the central lay up of the mainshaft would be ±28° in a 2.45 mm thick layer overlying an inner layer offibres wound at +89° in a 0.2 mm thick layer. Standard grade carbonfibre is used throughout. An epoxy bisphenol A resin with an anhydridecuring agent would typically be used as the matrix resin. This is usedto impregnate the fibre tows prior to laying down onto the mandrel. Thelongitudinal modulus of this construction is >50 GPa and the compositedensity is ˜1560 kgm⁻³. The taper angle of the end region is machined to4.9° with respect to the shaft axis to project the edges of the throughthickness plies.

The hoop stiffened end plugs are wound using similar processes andmaterials to those used in the main tube. The tubular plugs are wound(±89° (0.75 mm) ±20° (0.35 mm) )₂ to give a 21.3 mm as moulded internaldiameter with the outer diameter machined to 27.7 mm to give an lightinterference fit with the internal diameter of the main shaft. Thesetubes are push fit into the main shaft ends to a distance of 35 mm andthe protruding ends of the plug are machined back to match the ends ofthe main shaft.

The metallic sleeve would typically be based on a 1.35 mm thick thinwalled high strength steel with an internal diameter of 33.4 mm oflength 38 mm. The internal surface of the end fitting has 68 teeth witha spline length of 31 mm and an inboard length of 6 mm. The two splineends are simultaneously pushed onto the composite main shaft through theapplication of a compressive load of 30-40 kN. This creates a heavyinterference and in so doing cuts fine splined grooves into the externalsurface of the shaft. Testing of the serrated mechanical interface soproduced without any secondary bonding has shown that the mechanicaljoint is able to support torsional loads of in excess of 1200 Nm andtensile loads in excess of 25 kN.

A second embodiment of the invention consists of a similar primary jointas cited in the first example except that during the mechanical assemblyprocess an epoxy paste adhesive is incorporated into and spread aroundboth the 31 mm long mechanical interface defined by Ls and the 6 mm longregion defined by La along which a 0.15 mm thick adhesive layer can beaccommodated to protect the jointed area from the environment andoptimise the bond performance. The incorporation of the secondary loadpath enabled enhanced torsional strengths in excess of 1300 Nm andhigher tensile loads in excess of 30 kN to be supported. The adhesivealso seals the ends and machined surfaces of the shaft in the jointregion.

Although the invention has been described in connection with a preferredembodiment thereof, it will be appreciated by those skilled in the artthat additions, modifications, substitutions and deletions notspecifically described may be made without departing from the spirit andscope of the invention as defined in the appended claims. As such thisinvention is not restricted to the details of the foregoing example.

1. A method for forming a fibre reinforced composite shaft bearing ametallic flanged end coupling attached to the outside diameter of theshaft through a concentric cylindrical torsional joint, comprising thesteps of: a)—forming a wedge shaped inner layer of fibre (Hp)impregnated with a matrix material over a part of a cylindrical mandrel,the wedge shaped inner layer having a thick end and a thin end;b)—forming a layer of outer helical plies of fibre layers (He)impregnated with a matrix material extending for at least the length ofthe shaft over both the wedge shaped inner layer and at least part ofthe length of the cylindrical mandrel beyond the thin end of the wedgeshaped inner layer; c)—curing the matrix material to form a fibrereinforced composite shaft having a cylindrical part over most of itslength, and a tapered end part where the outer helical plies lie overthe wedge shaped inner layer; d)—machining the outer surface of at leastthe tapered end part of the composite shaft to form a cylindrical endregion, whereby the outer helical plies of fibre layers are exposed onthe outer surface of the cylindrical end region; and e)—attaching themetallic flanged end coupling to the outer surface of the cylindricalend region.
 2. A method according to claim 1, further comprising thestep of, prior to step b, forming an inner layer of fibre impregnatedwith a matrix material, at a high helical angle, over the mandrel forthe length of the shaft.
 3. A method according to claim 1, wherein thewedge shaped inner layer of fibre comprises fibre impregnated with amatrix material, at a high helical angle.
 4. A method according to claim2, wherein the high helical angle is in the range 75-89.9° with respectto the axis (C/L) of the mandrel.
 5. A method according to claim 4wherein the high helical angle is in the range 85-89.9° with respect tothe axis (C/L) of the mandrel.
 6. A method according to claim 1, whereinthe metallic flanged end coupling is coupled to the outer surface of thecylindrical end regions by a splined mechanical joint, thereby enablingtransfer of torsional, axial tensile and compressive loads from themetallic flanged end coupling directly through the outer helical pliesof fibre layers.
 7. A method according to claim 1, wherein at least twoof the outer helical plies of fibre layers are wound at differinghelical angles.
 8. A method according to claim 1, wherein at least twoof the outer helical plies of fibre layers are of differing fibrereinforcement materials.
 9. A method according to claim 1, where themetallic flanged end coupling is attached by an interference fit withthe cylindrical end region, and the metallic flanged end couplingcomprises a metallic serrated splined fitting having a multiplicity ofpointed teeth (Te) which cut into and form a mechanical interface witheach helical ply layer in the cylindrical end region.
 10. A methodaccording to claim 9, further comprising the step of inserting a tubularplug (Pg) into cylindrical end region, thereby to increase the hoopstiffness and strength of the cylindrical end region and to prevent thematerial of the cylindrical end region from buckling during theinterference fitting of the metallic flanged end coupling.
 11. A methodaccording to claim 10 wherein, after insertion of the plug, the plug ismachined to length such that the plug is coterminous with thecylindrical end region.
 12. A method according to claim 10, wherein theplug is formed from a multiplicity of filament wound layers ofreinforcement at distinct winding angles (±a2, ±a3).
 13. A methodaccording to claim 10, wherein the plug is machined on its outsidediameter (ODp) to give a light interference fit with an inner diameterof the cylindrical end region.
 14. A method according to claim 10,wherein at least two of the filament wound layers of the plug are ofdiffering fibre reinforcement material.
 15. A method according to claim1, further comprising the step of incorporating an adhesive bond betweenan outside diameter of the cylindrical end region and the metallicflanged end coupling.
 16. A method according to claim 15 where themetallic flanged end coupling is designed to accommodate an adhesivefillet.
 17. A fibre reinforced composite shaft bearing a metallicflanged end coupling attached to the outside diameter of the shaftthrough a concentric cylindrical torsional joint, comprising: a)—acylindrical end region comprising a wedge shaped inner layer of fibre(Hp) impregnated with a matrix material; b)—a layer of outer helicalplies of fibre layers (He) impregnated with a matrix material formingthe shaft and extending over the wedge shaped inner layer, wherein thelayer of outer helical plies has a tapered end part overlying wedgeshaped inner layer to form the cylindrical end portion, and wherein theouter helical plies of fibre layers are exposed on an outer surface ofthe cylindrical end region; and c)—the metallic flanged end couplingattached to the outer surface of the cylindrical end region.
 18. A fibrereinforced composite shaft according to claim 17, further comprising aninner layer of fibre impregnated with a matrix material, at a highhelical angle, over the length of the shaft.
 19. A fibre reinforcedcomposite shaft according to claim 17, wherein the wedge shaped innerlayer of fibre comprises fibre impregnated with a matrix material, at ahigh helical angle.
 20. A fibre reinforced composite shaft according toclaim 18, wherein the high helical angle is in the range 75-89.9° withrespect to the axis (C/L) of the shaft.
 21. A fibre reinforced compositeshaft according to claim 20 wherein the high helical angle is in therange 85-89.9° with respect to the axis (C/L) of the shaft.
 22. A fibrereinforced composite shaft according to claim 17, wherein the metallicflanged end coupling is coupled to the outer surface of the cylindricalend regions by a splined mechanical joint, thereby enabling transfer oftorsional, axial tensile and compressive loads from the metallic flangedend coupling directly through the outer helical plies of fibre layers.23. A fibre reinforced composite shaft according to claim 17, wherein atleast two of the outer helical plies of fibre layers are wound atdiffering helical angles.
 24. A fibre reinforced composite shaftaccording to claim 17, wherein at least two of the outer helical pliesof fibre layers are of differing fibre reinforcement materials.
 25. Afibre reinforced composite shaft according to claim 17, where themetallic flanged end coupling is attached by an interference fit withthe cylindrical end region, and the metallic flanged end couplingcomprises a metallic serrated splined fitting having a multiplicity ofpointed teeth (Te) which cut into and form a mechanical interface witheach helical ply layer in the cylindrical end region.
 26. A fibrereinforced composite shaft according to claim 17, further comprising atubular plug (Pg) inside the cylindrical end region, for increasing thehoop stiffness and strength of the cylindrical end region and forpreventing the material of the cylindrical end region from bucklingduring interference fitting of the metallic flanged end coupling.
 27. Afibre reinforced composite shaft according to claim 26 wherein the plugis coterminous with the cylindrical end region.
 28. A fibre reinforcedcomposite shaft according to claim 26, wherein the plug comprises amultiplicity of filament wound layers of reinforcement at distinctwinding angles (±a2, ±a3), and the plug gives a light interference fitwith an inner diameter of the cylindrical end region.
 29. A fibrereinforced composite shaft according to claim 17, wherein at least twoof the filament wound layers are of differing fibre reinforcementmaterial.
 30. A fibre reinforced composite shaft according to claim 17,further comprising an adhesive bond between an outside diameter of thecylindrical end region and the metallic flanged end coupling.
 31. Afibre reinforced composite shaft according to claim 30 where themetallic flanged end coupling accommodates an adhesive fillet. 32.(canceled)